Ultrasonic sensors for continuously or intermittently measuring the level (height) of a liquid within a vessel are well known. The liquid can be of any type including different types of chemical and high purity liquids used in the manufacture of semiconductors. Such sensors have a transducer located close to the bottom of the vessel that transmits a beam of ultrasonic energy upwardly to the liquid surface. The sensor transducer is most often mounted at the bottom end of an elongated probe that extends within the vessel. The ultrasonic energy is transmitted from the bottom end of the probe and is reflected from the liquid surface and received back at the transducer. The round trip transit time of the beam of energy is measured and if the velocity of the ultrasonic energy in the liquid is known the vessel's liquid level can be determined.
FIG. 1 shows the construction of a typical prior art bottom-up contact type sensor of the type discussed above. There is a vessel 12 of a material that is compatible with the liquid 14 that it contains. The vessel 12 can be of any height as needed for the application that it serves and has a top wall 16 on which a header 19 of the ultrasonic sensor 20 is mounted. The mounting arrangement shown is illustrative and the sensor 20 can be suspended from the outer edge of the vessel. A probe 22 extends downward from the header 19 into the vessel. The probe 22 is most often a cylindrical tube of a material that is compatible with the liquid 14. Stainless steel is suitable probe material for many applications although probes of different types of plastic also are used. The probe 22 has a vent hole 23 near its upper end somewhat below the vessel top wall 16.
An inner tube 24 also is suspended from the header 19 and extends to or somewhat past the lower end of the probe 22. The tube 24 also can be of stainless steel or plastic. A housing 27, typically of stainless steel or plastic, is attached to the lower end of tube 24 that contains the transducer element 26. Transducer 26 usually is of a piezoelectric ceramic material such as PZT (lead-zirconate-titanate). There is a space 21 between the housing 27 and the lower end of probe 22 so that the vessel liquid 14 can enter the probe 22 and rise to the same level as the liquid in the vessel. Wires 30 extend from an electronic module 34 outside of the vessel, through the header 19 and within the tube 24 to the transducer 26. The electronic module 34, which also can be mounted on the header 19, contains the necessary conventional components and circuits to provide pulses or bursts of electrical energy to the transducer 26. The transducer converts the electrical energy into ultrasonic (electromechanical) energy and transmits this energy upwardly (vertically as shown in FIG. 1) in the probe 22 toward the header 19. The upwardly transmitted ultrasonic energy is reflected from the interface of the liquid 14 and air, or other gas, downwardly back to the transducer 26 which converts the received ultrasonic energy back into electrical energy signals that are supplied to the electronics module 34. The circuits in the electronics module 34 includes a microprocessor that controls production of periodic pulses or bursts of pulses of electrical energy by a power amplifier that are supplied to the transducer 26. The microprocessor also computes the round trip time of the transmitted and received reflected ultrasonic energy in the probe 22 and from this calculates the level of the liquid in the probe 22 which is the level of the liquid in the vessel 12. The electronics module also can have a display which reads out the level measured. The module 34 also can transmit measurement information to another device such as a control circuit that turns a supply of liquid to the vessel on and off.
While the prior art sensor 20 of FIG. 1 is operative it has several disadvantages. In a typical sensor the housing 27 requires a minimum height of ⅜″ to ½ to encapsulate the transducer 26. This means that the level of the liquid 14 in the vessel cannot be measured below the housing height. Sometimes the vessel is constructed with a well below its bottom wall into which the housing 27 can extend to eliminate the measuring problem caused by the housing height. But this adds expense in the construction of the vessel and can make its mounting more difficult. Also if liquid stays on the top portion of housing 27 which encapsulate the transducer 26 this may provide a false indication. Another disadvantage is that a characteristic of PZT material is such that it “rings” when an energy pulse or burst of pulses is applied to it. The ringing time is as long as 30/40 microseconds. Because of the ringing, as the transducer transmits energy in a direction vertically upward along the probe longitudinal axis a “dead zone”, shown by the lines D-D, occurs between the upper end of the transducer housing 27 and a point within the probe 22. The duration of the transducer ringing does not allow level measurement in the “dead zone”, which can of a height as large as ¾″ to 1″.
Because of the problems described above the prior art sensor 20 is unable to measure the liquid level to the bottom of the vessel. Accordingly, it would be desirable to provide a sensor that can overcome the disadvantages of the prior art bottom up type sensors so that a more accurate measurement of liquid closer to the bottom of the vessel can be made.